Environmental Engineering Reference
In-Depth Information
produced. In order to reduce costs it is thus imperative to recover the acid aqueous
stream after pretreatment.
From investigations of the aqueous stream, after the solid pre-treated biomass
had been separated, it was found that a significant percentage of the organic acid
was lost from the aqueous phase (with respect to input), thus suggesting it is inti-
mately attached to the straw. Indeed when the solid pre-treated straw was used in the
subsequent aqueous enzymatic hydrolysis procedure, it was found that the remain-
ing acid was detected in solution. To what extent the presence of organic acid in this
media would have on the following fermentation process still remains to be inves-
tigated. However it does raise the issue, one acid solution recovery (directly after
pretreatment) or two?
2.5 Enzymatic Hydrolysis
Within this study readily available cellulase from Genencor was utilised under the
most optimal conditions that they recommend. No attempt to optimise these condi-
tions were attempted, as the greatest contribution to improving this process step is
the enzyme (and its production) itself and its price tag. This is an issue that producers
such as Novozymes and Genencor are tackling.
2.6 Adding Value to Rest Streams
During the conversion process some organic material remains. Is it possible to use
this for application as ruminant feed? Straw is not a significant source of dietary
protein and its value as ruminant feed is dependant on the carbohydrate content.
Very effective pretreatments of straw, as in the case of the use of sulphuric acid,
would result in a rest stream low in carbohydrates and contain amounts of furfural
and sulphate residues, making it unsuitable for animal feed. In the case of fumaric
acid pre-treated straw some carbohydrates would still be present. The presence of
non-recovered fumaric acid may also have some extra significance. In vitro stud-
ies have shown that methane emissions may be lowered [28, 29], thus fumarate in
ruminant feed could prove a positive addition.
So to answer the original questions
pretreatments of lignocellulosic materials
at elevated temperatures do not only need to be carried out using sulphuric acid.
It is possible, under the same conditions, to achieve high glucose yields with other
acids, for example with the use of less corrosive (organic) maleic and fumaric acid
that also results in less degradation of any liberated free sugars. However the use
of organic acids incurs more costs than processes operating on sulphuric acid, so
recovery and re-use of acid rest streams requires further attention. However it can
be mentioned that the price of sulphuric acid has risen significantly in the last year
from $90 to $329 per tonne [30]. It has been suggested that such increases have
been perpetuated by the rise in bioethanol production. However given the very large
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